Add m-esr52 at 52.6.0

4 files changed, 1800 insertions, 0 deletions

diff --git a/python/pytest/_pytest/assertion/__init__.py b/python/pytest/_pytest/assertion/__init__.py
new file mode 100644
index 000000000..6921deb2a
--- /dev/null
+++ b/python/pytest/_pytest/assertion/__init__.py
@@ -0,0 +1,176 @@
+"""
+support for presenting detailed information in failing assertions.
+"""
+import py
+import os
+import sys
+from _pytest.monkeypatch import monkeypatch
+from _pytest.assertion import util
+
+
+def pytest_addoption(parser):
+    group = parser.getgroup("debugconfig")
+    group.addoption('--assert',
+                    action="store",
+                    dest="assertmode",
+                    choices=("rewrite", "reinterp", "plain",),
+                    default="rewrite",
+                    metavar="MODE",
+                    help="""control assertion debugging tools.  'plain'
+                            performs no assertion debugging.  'reinterp'
+                            reinterprets assert statements after they failed
+                            to provide assertion expression information.
+                            'rewrite' (the default) rewrites assert
+                            statements in test modules on import to
+                            provide assert expression information. """)
+    group.addoption('--no-assert',
+                    action="store_true",
+                    default=False,
+                    dest="noassert",
+                    help="DEPRECATED equivalent to --assert=plain")
+    group.addoption('--nomagic', '--no-magic',
+                    action="store_true",
+                    default=False,
+                    help="DEPRECATED equivalent to --assert=plain")
+
+
+class AssertionState:
+    """State for the assertion plugin."""
+
+    def __init__(self, config, mode):
+        self.mode = mode
+        self.trace = config.trace.root.get("assertion")
+
+
+def pytest_configure(config):
+    mode = config.getvalue("assertmode")
+    if config.getvalue("noassert") or config.getvalue("nomagic"):
+        mode = "plain"
+    if mode == "rewrite":
+        try:
+            import ast  # noqa
+        except ImportError:
+            mode = "reinterp"
+        else:
+            # Both Jython and CPython 2.6.0 have AST bugs that make the
+            # assertion rewriting hook malfunction.
+            if (sys.platform.startswith('java') or
+                    sys.version_info[:3] == (2, 6, 0)):
+                mode = "reinterp"
+    if mode != "plain":
+        _load_modules(mode)
+        m = monkeypatch()
+        config._cleanup.append(m.undo)
+        m.setattr(py.builtin.builtins, 'AssertionError',
+                  reinterpret.AssertionError)  # noqa
+    hook = None
+    if mode == "rewrite":
+        hook = rewrite.AssertionRewritingHook()  # noqa
+        sys.meta_path.insert(0, hook)
+    warn_about_missing_assertion(mode)
+    config._assertstate = AssertionState(config, mode)
+    config._assertstate.hook = hook
+    config._assertstate.trace("configured with mode set to %r" % (mode,))
+    def undo():
+        hook = config._assertstate.hook
+        if hook is not None and hook in sys.meta_path:
+            sys.meta_path.remove(hook)
+    config.add_cleanup(undo)
+
+
+def pytest_collection(session):
+    # this hook is only called when test modules are collected
+    # so for example not in the master process of pytest-xdist
+    # (which does not collect test modules)
+    hook = session.config._assertstate.hook
+    if hook is not None:
+        hook.set_session(session)
+
+
+def _running_on_ci():
+    """Check if we're currently running on a CI system."""
+    env_vars = ['CI', 'BUILD_NUMBER']
+    return any(var in os.environ for var in env_vars)
+
+
+def pytest_runtest_setup(item):
+    """Setup the pytest_assertrepr_compare hook
+
+    The newinterpret and rewrite modules will use util._reprcompare if
+    it exists to use custom reporting via the
+    pytest_assertrepr_compare hook.  This sets up this custom
+    comparison for the test.
+    """
+    def callbinrepr(op, left, right):
+        """Call the pytest_assertrepr_compare hook and prepare the result
+
+        This uses the first result from the hook and then ensures the
+        following:
+        * Overly verbose explanations are dropped unless -vv was used or
+          running on a CI.
+        * Embedded newlines are escaped to help util.format_explanation()
+          later.
+        * If the rewrite mode is used embedded %-characters are replaced
+          to protect later % formatting.
+
+        The result can be formatted by util.format_explanation() for
+        pretty printing.
+        """
+        hook_result = item.ihook.pytest_assertrepr_compare(
+            config=item.config, op=op, left=left, right=right)
+        for new_expl in hook_result:
+            if new_expl:
+                if (sum(len(p) for p in new_expl[1:]) > 80*8 and
+                        item.config.option.verbose < 2 and
+                        not _running_on_ci()):
+                    show_max = 10
+                    truncated_lines = len(new_expl) - show_max
+                    new_expl[show_max:] = [py.builtin._totext(
+                        'Detailed information truncated (%d more lines)'
+                        ', use "-vv" to show' % truncated_lines)]
+                new_expl = [line.replace("\n", "\\n") for line in new_expl]
+                res = py.builtin._totext("\n~").join(new_expl)
+                if item.config.getvalue("assertmode") == "rewrite":
+                    res = res.replace("%", "%%")
+                return res
+    util._reprcompare = callbinrepr
+
+
+def pytest_runtest_teardown(item):
+    util._reprcompare = None
+
+
+def pytest_sessionfinish(session):
+    hook = session.config._assertstate.hook
+    if hook is not None:
+        hook.session = None
+
+
+def _load_modules(mode):
+    """Lazily import assertion related code."""
+    global rewrite, reinterpret
+    from _pytest.assertion import reinterpret  # noqa
+    if mode == "rewrite":
+        from _pytest.assertion import rewrite  # noqa
+
+
+def warn_about_missing_assertion(mode):
+    try:
+        assert False
+    except AssertionError:
+        pass
+    else:
+        if mode == "rewrite":
+            specifically = ("assertions which are not in test modules "
+                            "will be ignored")
+        else:
+            specifically = "failing tests may report as passing"
+
+        sys.stderr.write("WARNING: " + specifically +
+                         " because assert statements are not executed "
+                         "by the underlying Python interpreter "
+                         "(are you using python -O?)\n")
+
+
+# Expose this plugin's implementation for the pytest_assertrepr_compare hook
+pytest_assertrepr_compare = util.assertrepr_compare
diff --git a/python/pytest/_pytest/assertion/reinterpret.py b/python/pytest/_pytest/assertion/reinterpret.py
new file mode 100644
index 000000000..f4262c3ac
--- /dev/null
+++ b/python/pytest/_pytest/assertion/reinterpret.py
@@ -0,0 +1,407 @@
+"""
+Find intermediate evalutation results in assert statements through builtin AST.
+"""
+import ast
+import sys
+
+import _pytest._code
+import py
+from _pytest.assertion import util
+u = py.builtin._totext
+
+
+class AssertionError(util.BuiltinAssertionError):
+    def __init__(self, *args):
+        util.BuiltinAssertionError.__init__(self, *args)
+        if args:
+            # on Python2.6 we get len(args)==2 for: assert 0, (x,y)
+            # on Python2.7 and above we always get len(args) == 1
+            # with args[0] being the (x,y) tuple.
+            if len(args) > 1:
+                toprint = args
+            else:
+                toprint = args[0]
+            try:
+                self.msg = u(toprint)
+            except Exception:
+                self.msg = u(
+                    "<[broken __repr__] %s at %0xd>"
+                    % (toprint.__class__, id(toprint)))
+        else:
+            f = _pytest._code.Frame(sys._getframe(1))
+            try:
+                source = f.code.fullsource
+                if source is not None:
+                    try:
+                        source = source.getstatement(f.lineno, assertion=True)
+                    except IndexError:
+                        source = None
+                    else:
+                        source = str(source.deindent()).strip()
+            except py.error.ENOENT:
+                source = None
+                # this can also occur during reinterpretation, when the
+                # co_filename is set to "<run>".
+            if source:
+                self.msg = reinterpret(source, f, should_fail=True)
+            else:
+                self.msg = "<could not determine information>"
+            if not self.args:
+                self.args = (self.msg,)
+
+if sys.version_info > (3, 0):
+    AssertionError.__module__ = "builtins"
+
+if sys.platform.startswith("java"):
+    # See http://bugs.jython.org/issue1497
+    _exprs = ("BoolOp", "BinOp", "UnaryOp", "Lambda", "IfExp", "Dict",
+              "ListComp", "GeneratorExp", "Yield", "Compare", "Call",
+              "Repr", "Num", "Str", "Attribute", "Subscript", "Name",
+              "List", "Tuple")
+    _stmts = ("FunctionDef", "ClassDef", "Return", "Delete", "Assign",
+              "AugAssign", "Print", "For", "While", "If", "With", "Raise",
+              "TryExcept", "TryFinally", "Assert", "Import", "ImportFrom",
+              "Exec", "Global", "Expr", "Pass", "Break", "Continue")
+    _expr_nodes = set(getattr(ast, name) for name in _exprs)
+    _stmt_nodes = set(getattr(ast, name) for name in _stmts)
+    def _is_ast_expr(node):
+        return node.__class__ in _expr_nodes
+    def _is_ast_stmt(node):
+        return node.__class__ in _stmt_nodes
+else:
+    def _is_ast_expr(node):
+        return isinstance(node, ast.expr)
+    def _is_ast_stmt(node):
+        return isinstance(node, ast.stmt)
+
+try:
+    _Starred = ast.Starred
+except AttributeError:
+    # Python 2. Define a dummy class so isinstance() will always be False.
+    class _Starred(object): pass
+
+
+class Failure(Exception):
+    """Error found while interpreting AST."""
+
+    def __init__(self, explanation=""):
+        self.cause = sys.exc_info()
+        self.explanation = explanation
+
+
+def reinterpret(source, frame, should_fail=False):
+    mod = ast.parse(source)
+    visitor = DebugInterpreter(frame)
+    try:
+        visitor.visit(mod)
+    except Failure:
+        failure = sys.exc_info()[1]
+        return getfailure(failure)
+    if should_fail:
+        return ("(assertion failed, but when it was re-run for "
+                "printing intermediate values, it did not fail.  Suggestions: "
+                "compute assert expression before the assert or use --assert=plain)")
+
+def run(offending_line, frame=None):
+    if frame is None:
+        frame = _pytest._code.Frame(sys._getframe(1))
+    return reinterpret(offending_line, frame)
+
+def getfailure(e):
+    explanation = util.format_explanation(e.explanation)
+    value = e.cause[1]
+    if str(value):
+        lines = explanation.split('\n')
+        lines[0] += "  << %s" % (value,)
+        explanation = '\n'.join(lines)
+    text = "%s: %s" % (e.cause[0].__name__, explanation)
+    if text.startswith('AssertionError: assert '):
+        text = text[16:]
+    return text
+
+operator_map = {
+    ast.BitOr : "|",
+    ast.BitXor : "^",
+    ast.BitAnd : "&",
+    ast.LShift : "<<",
+    ast.RShift : ">>",
+    ast.Add : "+",
+    ast.Sub : "-",
+    ast.Mult : "*",
+    ast.Div : "/",
+    ast.FloorDiv : "//",
+    ast.Mod : "%",
+    ast.Eq : "==",
+    ast.NotEq : "!=",
+    ast.Lt : "<",
+    ast.LtE : "<=",
+    ast.Gt : ">",
+    ast.GtE : ">=",
+    ast.Pow : "**",
+    ast.Is : "is",
+    ast.IsNot : "is not",
+    ast.In : "in",
+    ast.NotIn : "not in"
+}
+
+unary_map = {
+    ast.Not : "not %s",
+    ast.Invert : "~%s",
+    ast.USub : "-%s",
+    ast.UAdd : "+%s"
+}
+
+
+class DebugInterpreter(ast.NodeVisitor):
+    """Interpret AST nodes to gleam useful debugging information. """
+
+    def __init__(self, frame):
+        self.frame = frame
+
+    def generic_visit(self, node):
+        # Fallback when we don't have a special implementation.
+        if _is_ast_expr(node):
+            mod = ast.Expression(node)
+            co = self._compile(mod)
+            try:
+                result = self.frame.eval(co)
+            except Exception:
+                raise Failure()
+            explanation = self.frame.repr(result)
+            return explanation, result
+        elif _is_ast_stmt(node):
+            mod = ast.Module([node])
+            co = self._compile(mod, "exec")
+            try:
+                self.frame.exec_(co)
+            except Exception:
+                raise Failure()
+            return None, None
+        else:
+            raise AssertionError("can't handle %s" %(node,))
+
+    def _compile(self, source, mode="eval"):
+        return compile(source, "<assertion interpretation>", mode)
+
+    def visit_Expr(self, expr):
+        return self.visit(expr.value)
+
+    def visit_Module(self, mod):
+        for stmt in mod.body:
+            self.visit(stmt)
+
+    def visit_Name(self, name):
+        explanation, result = self.generic_visit(name)
+        # See if the name is local.
+        source = "%r in locals() is not globals()" % (name.id,)
+        co = self._compile(source)
+        try:
+            local = self.frame.eval(co)
+        except Exception:
+            # have to assume it isn't
+            local = None
+        if local is None or not self.frame.is_true(local):
+            return name.id, result
+        return explanation, result
+
+    def visit_Compare(self, comp):
+        left = comp.left
+        left_explanation, left_result = self.visit(left)
+        for op, next_op in zip(comp.ops, comp.comparators):
+            next_explanation, next_result = self.visit(next_op)
+            op_symbol = operator_map[op.__class__]
+            explanation = "%s %s %s" % (left_explanation, op_symbol,
+                                        next_explanation)
+            source = "__exprinfo_left %s __exprinfo_right" % (op_symbol,)
+            co = self._compile(source)
+            try:
+                result = self.frame.eval(co, __exprinfo_left=left_result,
+                                         __exprinfo_right=next_result)
+            except Exception:
+                raise Failure(explanation)
+            try:
+                if not self.frame.is_true(result):
+                    break
+            except KeyboardInterrupt:
+                raise
+            except:
+                break
+            left_explanation, left_result = next_explanation, next_result
+
+        if util._reprcompare is not None:
+            res = util._reprcompare(op_symbol, left_result, next_result)
+            if res:
+                explanation = res
+        return explanation, result
+
+    def visit_BoolOp(self, boolop):
+        is_or = isinstance(boolop.op, ast.Or)
+        explanations = []
+        for operand in boolop.values:
+            explanation, result = self.visit(operand)
+            explanations.append(explanation)
+            if result == is_or:
+                break
+        name = is_or and " or " or " and "
+        explanation = "(" + name.join(explanations) + ")"
+        return explanation, result
+
+    def visit_UnaryOp(self, unary):
+        pattern = unary_map[unary.op.__class__]
+        operand_explanation, operand_result = self.visit(unary.operand)
+        explanation = pattern % (operand_explanation,)
+        co = self._compile(pattern % ("__exprinfo_expr",))
+        try:
+            result = self.frame.eval(co, __exprinfo_expr=operand_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, result
+
+    def visit_BinOp(self, binop):
+        left_explanation, left_result = self.visit(binop.left)
+        right_explanation, right_result = self.visit(binop.right)
+        symbol = operator_map[binop.op.__class__]
+        explanation = "(%s %s %s)" % (left_explanation, symbol,
+                                      right_explanation)
+        source = "__exprinfo_left %s __exprinfo_right" % (symbol,)
+        co = self._compile(source)
+        try:
+            result = self.frame.eval(co, __exprinfo_left=left_result,
+                                     __exprinfo_right=right_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, result
+
+    def visit_Call(self, call):
+        func_explanation, func = self.visit(call.func)
+        arg_explanations = []
+        ns = {"__exprinfo_func" : func}
+        arguments = []
+        for arg in call.args:
+            arg_explanation, arg_result = self.visit(arg)
+            if isinstance(arg, _Starred):
+                arg_name = "__exprinfo_star"
+                ns[arg_name] = arg_result
+                arguments.append("*%s" % (arg_name,))
+                arg_explanations.append("*%s" % (arg_explanation,))
+            else:
+                arg_name = "__exprinfo_%s" % (len(ns),)
+                ns[arg_name] = arg_result
+                arguments.append(arg_name)
+                arg_explanations.append(arg_explanation)
+        for keyword in call.keywords:
+            arg_explanation, arg_result = self.visit(keyword.value)
+            if keyword.arg:
+                arg_name = "__exprinfo_%s" % (len(ns),)
+                keyword_source = "%s=%%s" % (keyword.arg)
+                arguments.append(keyword_source % (arg_name,))
+                arg_explanations.append(keyword_source % (arg_explanation,))
+            else:
+                arg_name = "__exprinfo_kwds"
+                arguments.append("**%s" % (arg_name,))
+                arg_explanations.append("**%s" % (arg_explanation,))
+
+            ns[arg_name] = arg_result
+
+        if getattr(call, 'starargs', None):
+            arg_explanation, arg_result = self.visit(call.starargs)
+            arg_name = "__exprinfo_star"
+            ns[arg_name] = arg_result
+            arguments.append("*%s" % (arg_name,))
+            arg_explanations.append("*%s" % (arg_explanation,))
+
+        if getattr(call, 'kwargs', None):
+            arg_explanation, arg_result = self.visit(call.kwargs)
+            arg_name = "__exprinfo_kwds"
+            ns[arg_name] = arg_result
+            arguments.append("**%s" % (arg_name,))
+            arg_explanations.append("**%s" % (arg_explanation,))
+        args_explained = ", ".join(arg_explanations)
+        explanation = "%s(%s)" % (func_explanation, args_explained)
+        args = ", ".join(arguments)
+        source = "__exprinfo_func(%s)" % (args,)
+        co = self._compile(source)
+        try:
+            result = self.frame.eval(co, **ns)
+        except Exception:
+            raise Failure(explanation)
+        pattern = "%s\n{%s = %s\n}"
+        rep = self.frame.repr(result)
+        explanation = pattern % (rep, rep, explanation)
+        return explanation, result
+
+    def _is_builtin_name(self, name):
+        pattern = "%r not in globals() and %r not in locals()"
+        source = pattern % (name.id, name.id)
+        co = self._compile(source)
+        try:
+            return self.frame.eval(co)
+        except Exception:
+            return False
+
+    def visit_Attribute(self, attr):
+        if not isinstance(attr.ctx, ast.Load):
+            return self.generic_visit(attr)
+        source_explanation, source_result = self.visit(attr.value)
+        explanation = "%s.%s" % (source_explanation, attr.attr)
+        source = "__exprinfo_expr.%s" % (attr.attr,)
+        co = self._compile(source)
+        try:
+            try:
+                result = self.frame.eval(co, __exprinfo_expr=source_result)
+            except AttributeError:
+                # Maybe the attribute name needs to be mangled?
+                if not attr.attr.startswith("__") or attr.attr.endswith("__"):
+                    raise
+                source = "getattr(__exprinfo_expr.__class__, '__name__', '')"
+                co = self._compile(source)
+                class_name = self.frame.eval(co, __exprinfo_expr=source_result)
+                mangled_attr = "_" + class_name +  attr.attr
+                source = "__exprinfo_expr.%s" % (mangled_attr,)
+                co = self._compile(source)
+                result = self.frame.eval(co, __exprinfo_expr=source_result)
+        except Exception:
+            raise Failure(explanation)
+        explanation = "%s\n{%s = %s.%s\n}" % (self.frame.repr(result),
+                                              self.frame.repr(result),
+                                              source_explanation, attr.attr)
+        # Check if the attr is from an instance.
+        source = "%r in getattr(__exprinfo_expr, '__dict__', {})"
+        source = source % (attr.attr,)
+        co = self._compile(source)
+        try:
+            from_instance = self.frame.eval(co, __exprinfo_expr=source_result)
+        except Exception:
+            from_instance = None
+        if from_instance is None or self.frame.is_true(from_instance):
+            rep = self.frame.repr(result)
+            pattern = "%s\n{%s = %s\n}"
+            explanation = pattern % (rep, rep, explanation)
+        return explanation, result
+
+    def visit_Assert(self, assrt):
+        test_explanation, test_result = self.visit(assrt.test)
+        explanation = "assert %s" % (test_explanation,)
+        if not self.frame.is_true(test_result):
+            try:
+                raise util.BuiltinAssertionError
+            except Exception:
+                raise Failure(explanation)
+        return explanation, test_result
+
+    def visit_Assign(self, assign):
+        value_explanation, value_result = self.visit(assign.value)
+        explanation = "... = %s" % (value_explanation,)
+        name = ast.Name("__exprinfo_expr", ast.Load(),
+                        lineno=assign.value.lineno,
+                        col_offset=assign.value.col_offset)
+        new_assign = ast.Assign(assign.targets, name, lineno=assign.lineno,
+                                col_offset=assign.col_offset)
+        mod = ast.Module([new_assign])
+        co = self._compile(mod, "exec")
+        try:
+            self.frame.exec_(co, __exprinfo_expr=value_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, value_result
+
diff --git a/python/pytest/_pytest/assertion/rewrite.py b/python/pytest/_pytest/assertion/rewrite.py
new file mode 100644
index 000000000..14b8e49db
--- /dev/null
+++ b/python/pytest/_pytest/assertion/rewrite.py
@@ -0,0 +1,885 @@
+"""Rewrite assertion AST to produce nice error messages"""
+
+import ast
+import errno
+import itertools
+import imp
+import marshal
+import os
+import re
+import struct
+import sys
+import types
+
+import py
+from _pytest.assertion import util
+
+
+# pytest caches rewritten pycs in __pycache__.
+if hasattr(imp, "get_tag"):
+    PYTEST_TAG = imp.get_tag() + "-PYTEST"
+else:
+    if hasattr(sys, "pypy_version_info"):
+        impl = "pypy"
+    elif sys.platform == "java":
+        impl = "jython"
+    else:
+        impl = "cpython"
+    ver = sys.version_info
+    PYTEST_TAG = "%s-%s%s-PYTEST" % (impl, ver[0], ver[1])
+    del ver, impl
+
+PYC_EXT = ".py" + (__debug__ and "c" or "o")
+PYC_TAIL = "." + PYTEST_TAG + PYC_EXT
+
+REWRITE_NEWLINES = sys.version_info[:2] != (2, 7) and sys.version_info < (3, 2)
+ASCII_IS_DEFAULT_ENCODING = sys.version_info[0] < 3
+
+if sys.version_info >= (3,5):
+    ast_Call = ast.Call
+else:
+    ast_Call = lambda a,b,c: ast.Call(a, b, c, None, None)
+
+
+class AssertionRewritingHook(object):
+    """PEP302 Import hook which rewrites asserts."""
+
+    def __init__(self):
+        self.session = None
+        self.modules = {}
+        self._register_with_pkg_resources()
+
+    def set_session(self, session):
+        self.fnpats = session.config.getini("python_files")
+        self.session = session
+
+    def find_module(self, name, path=None):
+        if self.session is None:
+            return None
+        sess = self.session
+        state = sess.config._assertstate
+        state.trace("find_module called for: %s" % name)
+        names = name.rsplit(".", 1)
+        lastname = names[-1]
+        pth = None
+        if path is not None:
+            # Starting with Python 3.3, path is a _NamespacePath(), which
+            # causes problems if not converted to list.
+            path = list(path)
+            if len(path) == 1:
+                pth = path[0]
+        if pth is None:
+            try:
+                fd, fn, desc = imp.find_module(lastname, path)
+            except ImportError:
+                return None
+            if fd is not None:
+                fd.close()
+            tp = desc[2]
+            if tp == imp.PY_COMPILED:
+                if hasattr(imp, "source_from_cache"):
+                    fn = imp.source_from_cache(fn)
+                else:
+                    fn = fn[:-1]
+            elif tp != imp.PY_SOURCE:
+                # Don't know what this is.
+                return None
+        else:
+            fn = os.path.join(pth, name.rpartition(".")[2] + ".py")
+        fn_pypath = py.path.local(fn)
+        # Is this a test file?
+        if not sess.isinitpath(fn):
+            # We have to be very careful here because imports in this code can
+            # trigger a cycle.
+            self.session = None
+            try:
+                for pat in self.fnpats:
+                    if fn_pypath.fnmatch(pat):
+                        state.trace("matched test file %r" % (fn,))
+                        break
+                else:
+                    return None
+            finally:
+                self.session = sess
+        else:
+            state.trace("matched test file (was specified on cmdline): %r" %
+                        (fn,))
+        # The requested module looks like a test file, so rewrite it. This is
+        # the most magical part of the process: load the source, rewrite the
+        # asserts, and load the rewritten source. We also cache the rewritten
+        # module code in a special pyc. We must be aware of the possibility of
+        # concurrent pytest processes rewriting and loading pycs. To avoid
+        # tricky race conditions, we maintain the following invariant: The
+        # cached pyc is always a complete, valid pyc. Operations on it must be
+        # atomic. POSIX's atomic rename comes in handy.
+        write = not sys.dont_write_bytecode
+        cache_dir = os.path.join(fn_pypath.dirname, "__pycache__")
+        if write:
+            try:
+                os.mkdir(cache_dir)
+            except OSError:
+                e = sys.exc_info()[1].errno
+                if e == errno.EEXIST:
+                    # Either the __pycache__ directory already exists (the
+                    # common case) or it's blocked by a non-dir node. In the
+                    # latter case, we'll ignore it in _write_pyc.
+                    pass
+                elif e in [errno.ENOENT, errno.ENOTDIR]:
+                    # One of the path components was not a directory, likely
+                    # because we're in a zip file.
+                    write = False
+                elif e in [errno.EACCES, errno.EROFS, errno.EPERM]:
+                    state.trace("read only directory: %r" % fn_pypath.dirname)
+                    write = False
+                else:
+                    raise
+        cache_name = fn_pypath.basename[:-3] + PYC_TAIL
+        pyc = os.path.join(cache_dir, cache_name)
+        # Notice that even if we're in a read-only directory, I'm going
+        # to check for a cached pyc. This may not be optimal...
+        co = _read_pyc(fn_pypath, pyc, state.trace)
+        if co is None:
+            state.trace("rewriting %r" % (fn,))
+            source_stat, co = _rewrite_test(state, fn_pypath)
+            if co is None:
+                # Probably a SyntaxError in the test.
+                return None
+            if write:
+                _make_rewritten_pyc(state, source_stat, pyc, co)
+        else:
+            state.trace("found cached rewritten pyc for %r" % (fn,))
+        self.modules[name] = co, pyc
+        return self
+
+    def load_module(self, name):
+        # If there is an existing module object named 'fullname' in
+        # sys.modules, the loader must use that existing module. (Otherwise,
+        # the reload() builtin will not work correctly.)
+        if name in sys.modules:
+            return sys.modules[name]
+
+        co, pyc = self.modules.pop(name)
+        # I wish I could just call imp.load_compiled here, but __file__ has to
+        # be set properly. In Python 3.2+, this all would be handled correctly
+        # by load_compiled.
+        mod = sys.modules[name] = imp.new_module(name)
+        try:
+            mod.__file__ = co.co_filename
+            # Normally, this attribute is 3.2+.
+            mod.__cached__ = pyc
+            mod.__loader__ = self
+            py.builtin.exec_(co, mod.__dict__)
+        except:
+            del sys.modules[name]
+            raise
+        return sys.modules[name]
+
+
+
+    def is_package(self, name):
+        try:
+            fd, fn, desc = imp.find_module(name)
+        except ImportError:
+            return False
+        if fd is not None:
+            fd.close()
+        tp = desc[2]
+        return tp == imp.PKG_DIRECTORY
+
+    @classmethod
+    def _register_with_pkg_resources(cls):
+        """
+        Ensure package resources can be loaded from this loader. May be called
+        multiple times, as the operation is idempotent.
+        """
+        try:
+            import pkg_resources
+            # access an attribute in case a deferred importer is present
+            pkg_resources.__name__
+        except ImportError:
+            return
+
+        # Since pytest tests are always located in the file system, the
+        #  DefaultProvider is appropriate.
+        pkg_resources.register_loader_type(cls, pkg_resources.DefaultProvider)
+
+    def get_data(self, pathname):
+        """Optional PEP302 get_data API.
+        """
+        with open(pathname, 'rb') as f:
+            return f.read()
+
+
+def _write_pyc(state, co, source_stat, pyc):
+    # Technically, we don't have to have the same pyc format as
+    # (C)Python, since these "pycs" should never be seen by builtin
+    # import. However, there's little reason deviate, and I hope
+    # sometime to be able to use imp.load_compiled to load them. (See
+    # the comment in load_module above.)
+    try:
+        fp = open(pyc, "wb")
+    except IOError:
+        err = sys.exc_info()[1].errno
+        state.trace("error writing pyc file at %s: errno=%s" %(pyc, err))
+        # we ignore any failure to write the cache file
+        # there are many reasons, permission-denied, __pycache__ being a
+        # file etc.
+        return False
+    try:
+        fp.write(imp.get_magic())
+        mtime = int(source_stat.mtime)
+        size = source_stat.size & 0xFFFFFFFF
+        fp.write(struct.pack("<ll", mtime, size))
+        marshal.dump(co, fp)
+    finally:
+        fp.close()
+    return True
+
+RN = "\r\n".encode("utf-8")
+N = "\n".encode("utf-8")
+
+cookie_re = re.compile(r"^[ \t\f]*#.*coding[:=][ \t]*[-\w.]+")
+BOM_UTF8 = '\xef\xbb\xbf'
+
+def _rewrite_test(state, fn):
+    """Try to read and rewrite *fn* and return the code object."""
+    try:
+        stat = fn.stat()
+        source = fn.read("rb")
+    except EnvironmentError:
+        return None, None
+    if ASCII_IS_DEFAULT_ENCODING:
+        # ASCII is the default encoding in Python 2. Without a coding
+        # declaration, Python 2 will complain about any bytes in the file
+        # outside the ASCII range. Sadly, this behavior does not extend to
+        # compile() or ast.parse(), which prefer to interpret the bytes as
+        # latin-1. (At least they properly handle explicit coding cookies.) To
+        # preserve this error behavior, we could force ast.parse() to use ASCII
+        # as the encoding by inserting a coding cookie. Unfortunately, that
+        # messes up line numbers. Thus, we have to check ourselves if anything
+        # is outside the ASCII range in the case no encoding is explicitly
+        # declared. For more context, see issue #269. Yay for Python 3 which
+        # gets this right.
+        end1 = source.find("\n")
+        end2 = source.find("\n", end1 + 1)
+        if (not source.startswith(BOM_UTF8) and
+            cookie_re.match(source[0:end1]) is None and
+            cookie_re.match(source[end1 + 1:end2]) is None):
+            if hasattr(state, "_indecode"):
+                # encodings imported us again, so don't rewrite.
+                return None, None
+            state._indecode = True
+            try:
+                try:
+                    source.decode("ascii")
+                except UnicodeDecodeError:
+                    # Let it fail in real import.
+                    return None, None
+            finally:
+                del state._indecode
+    # On Python versions which are not 2.7 and less than or equal to 3.1, the
+    # parser expects *nix newlines.
+    if REWRITE_NEWLINES:
+        source = source.replace(RN, N) + N
+    try:
+        tree = ast.parse(source)
+    except SyntaxError:
+        # Let this pop up again in the real import.
+        state.trace("failed to parse: %r" % (fn,))
+        return None, None
+    rewrite_asserts(tree)
+    try:
+        co = compile(tree, fn.strpath, "exec")
+    except SyntaxError:
+        # It's possible that this error is from some bug in the
+        # assertion rewriting, but I don't know of a fast way to tell.
+        state.trace("failed to compile: %r" % (fn,))
+        return None, None
+    return stat, co
+
+def _make_rewritten_pyc(state, source_stat, pyc, co):
+    """Try to dump rewritten code to *pyc*."""
+    if sys.platform.startswith("win"):
+        # Windows grants exclusive access to open files and doesn't have atomic
+        # rename, so just write into the final file.
+        _write_pyc(state, co, source_stat, pyc)
+    else:
+        # When not on windows, assume rename is atomic. Dump the code object
+        # into a file specific to this process and atomically replace it.
+        proc_pyc = pyc + "." + str(os.getpid())
+        if _write_pyc(state, co, source_stat, proc_pyc):
+            os.rename(proc_pyc, pyc)
+
+def _read_pyc(source, pyc, trace=lambda x: None):
+    """Possibly read a pytest pyc containing rewritten code.
+
+    Return rewritten code if successful or None if not.
+    """
+    try:
+        fp = open(pyc, "rb")
+    except IOError:
+        return None
+    with fp:
+        try:
+            mtime = int(source.mtime())
+            size = source.size()
+            data = fp.read(12)
+        except EnvironmentError as e:
+            trace('_read_pyc(%s): EnvironmentError %s' % (source, e))
+            return None
+        # Check for invalid or out of date pyc file.
+        if (len(data) != 12 or data[:4] != imp.get_magic() or
+                struct.unpack("<ll", data[4:]) != (mtime, size)):
+            trace('_read_pyc(%s): invalid or out of date pyc' % source)
+            return None
+        try:
+            co = marshal.load(fp)
+        except Exception as e:
+            trace('_read_pyc(%s): marshal.load error %s' % (source, e))
+            return None
+        if not isinstance(co, types.CodeType):
+            trace('_read_pyc(%s): not a code object' % source)
+            return None
+        return co
+
+
+def rewrite_asserts(mod):
+    """Rewrite the assert statements in mod."""
+    AssertionRewriter().run(mod)
+
+
+def _saferepr(obj):
+    """Get a safe repr of an object for assertion error messages.
+
+    The assertion formatting (util.format_explanation()) requires
+    newlines to be escaped since they are a special character for it.
+    Normally assertion.util.format_explanation() does this but for a
+    custom repr it is possible to contain one of the special escape
+    sequences, especially '\n{' and '\n}' are likely to be present in
+    JSON reprs.
+
+    """
+    repr = py.io.saferepr(obj)
+    if py.builtin._istext(repr):
+        t = py.builtin.text
+    else:
+        t = py.builtin.bytes
+    return repr.replace(t("\n"), t("\\n"))
+
+
+from _pytest.assertion.util import format_explanation as _format_explanation # noqa
+
+def _format_assertmsg(obj):
+    """Format the custom assertion message given.
+
+    For strings this simply replaces newlines with '\n~' so that
+    util.format_explanation() will preserve them instead of escaping
+    newlines.  For other objects py.io.saferepr() is used first.
+
+    """
+    # reprlib appears to have a bug which means that if a string
+    # contains a newline it gets escaped, however if an object has a
+    # .__repr__() which contains newlines it does not get escaped.
+    # However in either case we want to preserve the newline.
+    if py.builtin._istext(obj) or py.builtin._isbytes(obj):
+        s = obj
+        is_repr = False
+    else:
+        s = py.io.saferepr(obj)
+        is_repr = True
+    if py.builtin._istext(s):
+        t = py.builtin.text
+    else:
+        t = py.builtin.bytes
+    s = s.replace(t("\n"), t("\n~")).replace(t("%"), t("%%"))
+    if is_repr:
+        s = s.replace(t("\\n"), t("\n~"))
+    return s
+
+def _should_repr_global_name(obj):
+    return not hasattr(obj, "__name__") and not py.builtin.callable(obj)
+
+def _format_boolop(explanations, is_or):
+    explanation = "(" + (is_or and " or " or " and ").join(explanations) + ")"
+    if py.builtin._istext(explanation):
+        t = py.builtin.text
+    else:
+        t = py.builtin.bytes
+    return explanation.replace(t('%'), t('%%'))
+
+def _call_reprcompare(ops, results, expls, each_obj):
+    for i, res, expl in zip(range(len(ops)), results, expls):
+        try:
+            done = not res
+        except Exception:
+            done = True
+        if done:
+            break
+    if util._reprcompare is not None:
+        custom = util._reprcompare(ops[i], each_obj[i], each_obj[i + 1])
+        if custom is not None:
+            return custom
+    return expl
+
+
+unary_map = {
+    ast.Not: "not %s",
+    ast.Invert: "~%s",
+    ast.USub: "-%s",
+    ast.UAdd: "+%s"
+}
+
+binop_map = {
+    ast.BitOr: "|",
+    ast.BitXor: "^",
+    ast.BitAnd: "&",
+    ast.LShift: "<<",
+    ast.RShift: ">>",
+    ast.Add: "+",
+    ast.Sub: "-",
+    ast.Mult: "*",
+    ast.Div: "/",
+    ast.FloorDiv: "//",
+    ast.Mod: "%%", # escaped for string formatting
+    ast.Eq: "==",
+    ast.NotEq: "!=",
+    ast.Lt: "<",
+    ast.LtE: "<=",
+    ast.Gt: ">",
+    ast.GtE: ">=",
+    ast.Pow: "**",
+    ast.Is: "is",
+    ast.IsNot: "is not",
+    ast.In: "in",
+    ast.NotIn: "not in"
+}
+# Python 3.5+ compatibility
+try:
+    binop_map[ast.MatMult] = "@"
+except AttributeError:
+    pass
+
+# Python 3.4+ compatibility
+if hasattr(ast, "NameConstant"):
+    _NameConstant = ast.NameConstant
+else:
+    def _NameConstant(c):
+        return ast.Name(str(c), ast.Load())
+
+
+def set_location(node, lineno, col_offset):
+    """Set node location information recursively."""
+    def _fix(node, lineno, col_offset):
+        if "lineno" in node._attributes:
+            node.lineno = lineno
+        if "col_offset" in node._attributes:
+            node.col_offset = col_offset
+        for child in ast.iter_child_nodes(node):
+            _fix(child, lineno, col_offset)
+    _fix(node, lineno, col_offset)
+    return node
+
+
+class AssertionRewriter(ast.NodeVisitor):
+    """Assertion rewriting implementation.
+
+    The main entrypoint is to call .run() with an ast.Module instance,
+    this will then find all the assert statements and re-write them to
+    provide intermediate values and a detailed assertion error.  See
+    http://pybites.blogspot.be/2011/07/behind-scenes-of-pytests-new-assertion.html
+    for an overview of how this works.
+
+    The entry point here is .run() which will iterate over all the
+    statements in an ast.Module and for each ast.Assert statement it
+    finds call .visit() with it.  Then .visit_Assert() takes over and
+    is responsible for creating new ast statements to replace the
+    original assert statement: it re-writes the test of an assertion
+    to provide intermediate values and replace it with an if statement
+    which raises an assertion error with a detailed explanation in
+    case the expression is false.
+
+    For this .visit_Assert() uses the visitor pattern to visit all the
+    AST nodes of the ast.Assert.test field, each visit call returning
+    an AST node and the corresponding explanation string.  During this
+    state is kept in several instance attributes:
+
+    :statements: All the AST statements which will replace the assert
+       statement.
+
+    :variables: This is populated by .variable() with each variable
+       used by the statements so that they can all be set to None at
+       the end of the statements.
+
+    :variable_counter: Counter to create new unique variables needed
+       by statements.  Variables are created using .variable() and
+       have the form of "@py_assert0".
+
+    :on_failure: The AST statements which will be executed if the
+       assertion test fails.  This is the code which will construct
+       the failure message and raises the AssertionError.
+
+    :explanation_specifiers: A dict filled by .explanation_param()
+       with %-formatting placeholders and their corresponding
+       expressions to use in the building of an assertion message.
+       This is used by .pop_format_context() to build a message.
+
+    :stack: A stack of the explanation_specifiers dicts maintained by
+       .push_format_context() and .pop_format_context() which allows
+       to build another %-formatted string while already building one.
+
+    This state is reset on every new assert statement visited and used
+    by the other visitors.
+
+    """
+
+    def run(self, mod):
+        """Find all assert statements in *mod* and rewrite them."""
+        if not mod.body:
+            # Nothing to do.
+            return
+        # Insert some special imports at the top of the module but after any
+        # docstrings and __future__ imports.
+        aliases = [ast.alias(py.builtin.builtins.__name__, "@py_builtins"),
+                   ast.alias("_pytest.assertion.rewrite", "@pytest_ar")]
+        expect_docstring = True
+        pos = 0
+        lineno = 0
+        for item in mod.body:
+            if (expect_docstring and isinstance(item, ast.Expr) and
+                    isinstance(item.value, ast.Str)):
+                doc = item.value.s
+                if "PYTEST_DONT_REWRITE" in doc:
+                    # The module has disabled assertion rewriting.
+                    return
+                lineno += len(doc) - 1
+                expect_docstring = False
+            elif (not isinstance(item, ast.ImportFrom) or item.level > 0 or
+                  item.module != "__future__"):
+                lineno = item.lineno
+                break
+            pos += 1
+        imports = [ast.Import([alias], lineno=lineno, col_offset=0)
+                   for alias in aliases]
+        mod.body[pos:pos] = imports
+        # Collect asserts.
+        nodes = [mod]
+        while nodes:
+            node = nodes.pop()
+            for name, field in ast.iter_fields(node):
+                if isinstance(field, list):
+                    new = []
+                    for i, child in enumerate(field):
+                        if isinstance(child, ast.Assert):
+                            # Transform assert.
+                            new.extend(self.visit(child))
+                        else:
+                            new.append(child)
+                            if isinstance(child, ast.AST):
+                                nodes.append(child)
+                    setattr(node, name, new)
+                elif (isinstance(field, ast.AST) and
+                      # Don't recurse into expressions as they can't contain
+                      # asserts.
+                      not isinstance(field, ast.expr)):
+                    nodes.append(field)
+
+    def variable(self):
+        """Get a new variable."""
+        # Use a character invalid in python identifiers to avoid clashing.
+        name = "@py_assert" + str(next(self.variable_counter))
+        self.variables.append(name)
+        return name
+
+    def assign(self, expr):
+        """Give *expr* a name."""
+        name = self.variable()
+        self.statements.append(ast.Assign([ast.Name(name, ast.Store())], expr))
+        return ast.Name(name, ast.Load())
+
+    def display(self, expr):
+        """Call py.io.saferepr on the expression."""
+        return self.helper("saferepr", expr)
+
+    def helper(self, name, *args):
+        """Call a helper in this module."""
+        py_name = ast.Name("@pytest_ar", ast.Load())
+        attr = ast.Attribute(py_name, "_" + name, ast.Load())
+        return ast_Call(attr, list(args), [])
+
+    def builtin(self, name):
+        """Return the builtin called *name*."""
+        builtin_name = ast.Name("@py_builtins", ast.Load())
+        return ast.Attribute(builtin_name, name, ast.Load())
+
+    def explanation_param(self, expr):
+        """Return a new named %-formatting placeholder for expr.
+
+        This creates a %-formatting placeholder for expr in the
+        current formatting context, e.g. ``%(py0)s``.  The placeholder
+        and expr are placed in the current format context so that it
+        can be used on the next call to .pop_format_context().
+
+        """
+        specifier = "py" + str(next(self.variable_counter))
+        self.explanation_specifiers[specifier] = expr
+        return "%(" + specifier + ")s"
+
+    def push_format_context(self):
+        """Create a new formatting context.
+
+        The format context is used for when an explanation wants to
+        have a variable value formatted in the assertion message.  In
+        this case the value required can be added using
+        .explanation_param().  Finally .pop_format_context() is used
+        to format a string of %-formatted values as added by
+        .explanation_param().
+
+        """
+        self.explanation_specifiers = {}
+        self.stack.append(self.explanation_specifiers)
+
+    def pop_format_context(self, expl_expr):
+        """Format the %-formatted string with current format context.
+
+        The expl_expr should be an ast.Str instance constructed from
+        the %-placeholders created by .explanation_param().  This will
+        add the required code to format said string to .on_failure and
+        return the ast.Name instance of the formatted string.
+
+        """
+        current = self.stack.pop()
+        if self.stack:
+            self.explanation_specifiers = self.stack[-1]
+        keys = [ast.Str(key) for key in current.keys()]
+        format_dict = ast.Dict(keys, list(current.values()))
+        form = ast.BinOp(expl_expr, ast.Mod(), format_dict)
+        name = "@py_format" + str(next(self.variable_counter))
+        self.on_failure.append(ast.Assign([ast.Name(name, ast.Store())], form))
+        return ast.Name(name, ast.Load())
+
+    def generic_visit(self, node):
+        """Handle expressions we don't have custom code for."""
+        assert isinstance(node, ast.expr)
+        res = self.assign(node)
+        return res, self.explanation_param(self.display(res))
+
+    def visit_Assert(self, assert_):
+        """Return the AST statements to replace the ast.Assert instance.
+
+        This re-writes the test of an assertion to provide
+        intermediate values and replace it with an if statement which
+        raises an assertion error with a detailed explanation in case
+        the expression is false.
+
+        """
+        self.statements = []
+        self.variables = []
+        self.variable_counter = itertools.count()
+        self.stack = []
+        self.on_failure = []
+        self.push_format_context()
+        # Rewrite assert into a bunch of statements.
+        top_condition, explanation = self.visit(assert_.test)
+        # Create failure message.
+        body = self.on_failure
+        negation = ast.UnaryOp(ast.Not(), top_condition)
+        self.statements.append(ast.If(negation, body, []))
+        if assert_.msg:
+            assertmsg = self.helper('format_assertmsg', assert_.msg)
+            explanation = "\n>assert " + explanation
+        else:
+            assertmsg = ast.Str("")
+            explanation = "assert " + explanation
+        template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
+        msg = self.pop_format_context(template)
+        fmt = self.helper("format_explanation", msg)
+        err_name = ast.Name("AssertionError", ast.Load())
+        exc = ast_Call(err_name, [fmt], [])
+        if sys.version_info[0] >= 3:
+            raise_ = ast.Raise(exc, None)
+        else:
+            raise_ = ast.Raise(exc, None, None)
+        body.append(raise_)
+        # Clear temporary variables by setting them to None.
+        if self.variables:
+            variables = [ast.Name(name, ast.Store())
+                         for name in self.variables]
+            clear = ast.Assign(variables, _NameConstant(None))
+            self.statements.append(clear)
+        # Fix line numbers.
+        for stmt in self.statements:
+            set_location(stmt, assert_.lineno, assert_.col_offset)
+        return self.statements
+
+    def visit_Name(self, name):
+        # Display the repr of the name if it's a local variable or
+        # _should_repr_global_name() thinks it's acceptable.
+        locs = ast_Call(self.builtin("locals"), [], [])
+        inlocs = ast.Compare(ast.Str(name.id), [ast.In()], [locs])
+        dorepr = self.helper("should_repr_global_name", name)
+        test = ast.BoolOp(ast.Or(), [inlocs, dorepr])
+        expr = ast.IfExp(test, self.display(name), ast.Str(name.id))
+        return name, self.explanation_param(expr)
+
+    def visit_BoolOp(self, boolop):
+        res_var = self.variable()
+        expl_list = self.assign(ast.List([], ast.Load()))
+        app = ast.Attribute(expl_list, "append", ast.Load())
+        is_or = int(isinstance(boolop.op, ast.Or))
+        body = save = self.statements
+        fail_save = self.on_failure
+        levels = len(boolop.values) - 1
+        self.push_format_context()
+        # Process each operand, short-circuting if needed.
+        for i, v in enumerate(boolop.values):
+            if i:
+                fail_inner = []
+                # cond is set in a prior loop iteration below
+                self.on_failure.append(ast.If(cond, fail_inner, [])) # noqa
+                self.on_failure = fail_inner
+            self.push_format_context()
+            res, expl = self.visit(v)
+            body.append(ast.Assign([ast.Name(res_var, ast.Store())], res))
+            expl_format = self.pop_format_context(ast.Str(expl))
+            call = ast_Call(app, [expl_format], [])
+            self.on_failure.append(ast.Expr(call))
+            if i < levels:
+                cond = res
+                if is_or:
+                    cond = ast.UnaryOp(ast.Not(), cond)
+                inner = []
+                self.statements.append(ast.If(cond, inner, []))
+                self.statements = body = inner
+        self.statements = save
+        self.on_failure = fail_save
+        expl_template = self.helper("format_boolop", expl_list, ast.Num(is_or))
+        expl = self.pop_format_context(expl_template)
+        return ast.Name(res_var, ast.Load()), self.explanation_param(expl)
+
+    def visit_UnaryOp(self, unary):
+        pattern = unary_map[unary.op.__class__]
+        operand_res, operand_expl = self.visit(unary.operand)
+        res = self.assign(ast.UnaryOp(unary.op, operand_res))
+        return res, pattern % (operand_expl,)
+
+    def visit_BinOp(self, binop):
+        symbol = binop_map[binop.op.__class__]
+        left_expr, left_expl = self.visit(binop.left)
+        right_expr, right_expl = self.visit(binop.right)
+        explanation = "(%s %s %s)" % (left_expl, symbol, right_expl)
+        res = self.assign(ast.BinOp(left_expr, binop.op, right_expr))
+        return res, explanation
+
+    def visit_Call_35(self, call):
+        """
+        visit `ast.Call` nodes on Python3.5 and after
+        """
+        new_func, func_expl = self.visit(call.func)
+        arg_expls = []
+        new_args = []
+        new_kwargs = []
+        for arg in call.args:
+            res, expl = self.visit(arg)
+            arg_expls.append(expl)
+            new_args.append(res)
+        for keyword in call.keywords:
+            res, expl = self.visit(keyword.value)
+            new_kwargs.append(ast.keyword(keyword.arg, res))
+            if keyword.arg:
+                arg_expls.append(keyword.arg + "=" + expl)
+            else: ## **args have `arg` keywords with an .arg of None
+                arg_expls.append("**" + expl)
+
+        expl = "%s(%s)" % (func_expl, ', '.join(arg_expls))
+        new_call = ast.Call(new_func, new_args, new_kwargs)
+        res = self.assign(new_call)
+        res_expl = self.explanation_param(self.display(res))
+        outer_expl = "%s\n{%s = %s\n}" % (res_expl, res_expl, expl)
+        return res, outer_expl
+
+    def visit_Starred(self, starred):
+        # From Python 3.5, a Starred node can appear in a function call
+        res, expl = self.visit(starred.value)
+        return starred, '*' + expl
+
+    def visit_Call_legacy(self, call):
+        """
+        visit `ast.Call nodes on 3.4 and below`
+        """
+        new_func, func_expl = self.visit(call.func)
+        arg_expls = []
+        new_args = []
+        new_kwargs = []
+        new_star = new_kwarg = None
+        for arg in call.args:
+            res, expl = self.visit(arg)
+            new_args.append(res)
+            arg_expls.append(expl)
+        for keyword in call.keywords:
+            res, expl = self.visit(keyword.value)
+            new_kwargs.append(ast.keyword(keyword.arg, res))
+            arg_expls.append(keyword.arg + "=" + expl)
+        if call.starargs:
+            new_star, expl = self.visit(call.starargs)
+            arg_expls.append("*" + expl)
+        if call.kwargs:
+            new_kwarg, expl = self.visit(call.kwargs)
+            arg_expls.append("**" + expl)
+        expl = "%s(%s)" % (func_expl, ', '.join(arg_expls))
+        new_call = ast.Call(new_func, new_args, new_kwargs,
+                            new_star, new_kwarg)
+        res = self.assign(new_call)
+        res_expl = self.explanation_param(self.display(res))
+        outer_expl = "%s\n{%s = %s\n}" % (res_expl, res_expl, expl)
+        return res, outer_expl
+
+    # ast.Call signature changed on 3.5,
+    # conditionally change  which methods is named
+    # visit_Call depending on Python version
+    if sys.version_info >= (3, 5):
+        visit_Call = visit_Call_35
+    else:
+        visit_Call = visit_Call_legacy
+
+
+    def visit_Attribute(self, attr):
+        if not isinstance(attr.ctx, ast.Load):
+            return self.generic_visit(attr)
+        value, value_expl = self.visit(attr.value)
+        res = self.assign(ast.Attribute(value, attr.attr, ast.Load()))
+        res_expl = self.explanation_param(self.display(res))
+        pat = "%s\n{%s = %s.%s\n}"
+        expl = pat % (res_expl, res_expl, value_expl, attr.attr)
+        return res, expl
+
+    def visit_Compare(self, comp):
+        self.push_format_context()
+        left_res, left_expl = self.visit(comp.left)
+        res_variables = [self.variable() for i in range(len(comp.ops))]
+        load_names = [ast.Name(v, ast.Load()) for v in res_variables]
+        store_names = [ast.Name(v, ast.Store()) for v in res_variables]
+        it = zip(range(len(comp.ops)), comp.ops, comp.comparators)
+        expls = []
+        syms = []
+        results = [left_res]
+        for i, op, next_operand in it:
+            next_res, next_expl = self.visit(next_operand)
+            results.append(next_res)
+            sym = binop_map[op.__class__]
+            syms.append(ast.Str(sym))
+            expl = "%s %s %s" % (left_expl, sym, next_expl)
+            expls.append(ast.Str(expl))
+            res_expr = ast.Compare(left_res, [op], [next_res])
+            self.statements.append(ast.Assign([store_names[i]], res_expr))
+            left_res, left_expl = next_res, next_expl
+        # Use pytest.assertion.util._reprcompare if that's available.
+        expl_call = self.helper("call_reprcompare",
+                                ast.Tuple(syms, ast.Load()),
+                                ast.Tuple(load_names, ast.Load()),
+                                ast.Tuple(expls, ast.Load()),
+                                ast.Tuple(results, ast.Load()))
+        if len(comp.ops) > 1:
+            res = ast.BoolOp(ast.And(), load_names)
+        else:
+            res = load_names[0]
+        return res, self.explanation_param(self.pop_format_context(expl_call))
diff --git a/python/pytest/_pytest/assertion/util.py b/python/pytest/_pytest/assertion/util.py
new file mode 100644
index 000000000..f2f23efea
--- /dev/null
+++ b/python/pytest/_pytest/assertion/util.py
@@ -0,0 +1,332 @@
+"""Utilities for assertion debugging"""
+import pprint
+
+import _pytest._code
+import py
+try:
+    from collections import Sequence
+except ImportError:
+    Sequence = list
+
+BuiltinAssertionError = py.builtin.builtins.AssertionError
+u = py.builtin._totext
+
+# The _reprcompare attribute on the util module is used by the new assertion
+# interpretation code and assertion rewriter to detect this plugin was
+# loaded and in turn call the hooks defined here as part of the
+# DebugInterpreter.
+_reprcompare = None
+
+
+# the re-encoding is needed for python2 repr
+# with non-ascii characters (see issue 877 and 1379)
+def ecu(s):
+    try:
+        return u(s, 'utf-8', 'replace')
+    except TypeError:
+        return s
+
+
+def format_explanation(explanation):
+    """This formats an explanation
+
+    Normally all embedded newlines are escaped, however there are
+    three exceptions: \n{, \n} and \n~.  The first two are intended
+    cover nested explanations, see function and attribute explanations
+    for examples (.visit_Call(), visit_Attribute()).  The last one is
+    for when one explanation needs to span multiple lines, e.g. when
+    displaying diffs.
+    """
+    explanation = ecu(explanation)
+    explanation = _collapse_false(explanation)
+    lines = _split_explanation(explanation)
+    result = _format_lines(lines)
+    return u('\n').join(result)
+
+
+def _collapse_false(explanation):
+    """Collapse expansions of False
+
+    So this strips out any "assert False\n{where False = ...\n}"
+    blocks.
+    """
+    where = 0
+    while True:
+        start = where = explanation.find("False\n{False = ", where)
+        if where == -1:
+            break
+        level = 0
+        prev_c = explanation[start]
+        for i, c in enumerate(explanation[start:]):
+            if prev_c + c == "\n{":
+                level += 1
+            elif prev_c + c == "\n}":
+                level -= 1
+                if not level:
+                    break
+            prev_c = c
+        else:
+            raise AssertionError("unbalanced braces: %r" % (explanation,))
+        end = start + i
+        where = end
+        if explanation[end - 1] == '\n':
+            explanation = (explanation[:start] + explanation[start+15:end-1] +
+                           explanation[end+1:])
+            where -= 17
+    return explanation
+
+
+def _split_explanation(explanation):
+    """Return a list of individual lines in the explanation
+
+    This will return a list of lines split on '\n{', '\n}' and '\n~'.
+    Any other newlines will be escaped and appear in the line as the
+    literal '\n' characters.
+    """
+    raw_lines = (explanation or u('')).split('\n')
+    lines = [raw_lines[0]]
+    for l in raw_lines[1:]:
+        if l and l[0] in ['{', '}', '~', '>']:
+            lines.append(l)
+        else:
+            lines[-1] += '\\n' + l
+    return lines
+
+
+def _format_lines(lines):
+    """Format the individual lines
+
+    This will replace the '{', '}' and '~' characters of our mini
+    formatting language with the proper 'where ...', 'and ...' and ' +
+    ...' text, taking care of indentation along the way.
+
+    Return a list of formatted lines.
+    """
+    result = lines[:1]
+    stack = [0]
+    stackcnt = [0]
+    for line in lines[1:]:
+        if line.startswith('{'):
+            if stackcnt[-1]:
+                s = u('and   ')
+            else:
+                s = u('where ')
+            stack.append(len(result))
+            stackcnt[-1] += 1
+            stackcnt.append(0)
+            result.append(u(' +') + u('  ')*(len(stack)-1) + s + line[1:])
+        elif line.startswith('}'):
+            stack.pop()
+            stackcnt.pop()
+            result[stack[-1]] += line[1:]
+        else:
+            assert line[0] in ['~', '>']
+            stack[-1] += 1
+            indent = len(stack) if line.startswith('~') else len(stack) - 1
+            result.append(u('  ')*indent + line[1:])
+    assert len(stack) == 1
+    return result
+
+
+# Provide basestring in python3
+try:
+    basestring = basestring
+except NameError:
+    basestring = str
+
+
+def assertrepr_compare(config, op, left, right):
+    """Return specialised explanations for some operators/operands"""
+    width = 80 - 15 - len(op) - 2  # 15 chars indentation, 1 space around op
+    left_repr = py.io.saferepr(left, maxsize=int(width/2))
+    right_repr = py.io.saferepr(right, maxsize=width-len(left_repr))
+
+    summary = u('%s %s %s') % (ecu(left_repr), op, ecu(right_repr))
+
+    issequence = lambda x: (isinstance(x, (list, tuple, Sequence)) and
+                            not isinstance(x, basestring))
+    istext = lambda x: isinstance(x, basestring)
+    isdict = lambda x: isinstance(x, dict)
+    isset = lambda x: isinstance(x, (set, frozenset))
+
+    def isiterable(obj):
+        try:
+            iter(obj)
+            return not istext(obj)
+        except TypeError:
+            return False
+
+    verbose = config.getoption('verbose')
+    explanation = None
+    try:
+        if op == '==':
+            if istext(left) and istext(right):
+                explanation = _diff_text(left, right, verbose)
+            else:
+                if issequence(left) and issequence(right):
+                    explanation = _compare_eq_sequence(left, right, verbose)
+                elif isset(left) and isset(right):
+                    explanation = _compare_eq_set(left, right, verbose)
+                elif isdict(left) and isdict(right):
+                    explanation = _compare_eq_dict(left, right, verbose)
+                if isiterable(left) and isiterable(right):
+                    expl = _compare_eq_iterable(left, right, verbose)
+                    if explanation is not None:
+                        explanation.extend(expl)
+                    else:
+                        explanation = expl
+        elif op == 'not in':
+            if istext(left) and istext(right):
+                explanation = _notin_text(left, right, verbose)
+    except Exception:
+        explanation = [
+            u('(pytest_assertion plugin: representation of details failed.  '
+              'Probably an object has a faulty __repr__.)'),
+            u(_pytest._code.ExceptionInfo())]
+
+    if not explanation:
+        return None
+
+    return [summary] + explanation
+
+
+def _diff_text(left, right, verbose=False):
+    """Return the explanation for the diff between text or bytes
+
+    Unless --verbose is used this will skip leading and trailing
+    characters which are identical to keep the diff minimal.
+
+    If the input are bytes they will be safely converted to text.
+    """
+    from difflib import ndiff
+    explanation = []
+    if isinstance(left, py.builtin.bytes):
+        left = u(repr(left)[1:-1]).replace(r'\n', '\n')
+    if isinstance(right, py.builtin.bytes):
+        right = u(repr(right)[1:-1]).replace(r'\n', '\n')
+    if not verbose:
+        i = 0  # just in case left or right has zero length
+        for i in range(min(len(left), len(right))):
+            if left[i] != right[i]:
+                break
+        if i > 42:
+            i -= 10                 # Provide some context
+            explanation = [u('Skipping %s identical leading '
+                             'characters in diff, use -v to show') % i]
+            left = left[i:]
+            right = right[i:]
+        if len(left) == len(right):
+            for i in range(len(left)):
+                if left[-i] != right[-i]:
+                    break
+            if i > 42:
+                i -= 10     # Provide some context
+                explanation += [u('Skipping %s identical trailing '
+                                  'characters in diff, use -v to show') % i]
+                left = left[:-i]
+                right = right[:-i]
+    explanation += [line.strip('\n')
+                    for line in ndiff(left.splitlines(),
+                                      right.splitlines())]
+    return explanation
+
+
+def _compare_eq_iterable(left, right, verbose=False):
+    if not verbose:
+        return [u('Use -v to get the full diff')]
+    # dynamic import to speedup pytest
+    import difflib
+
+    try:
+        left_formatting = pprint.pformat(left).splitlines()
+        right_formatting = pprint.pformat(right).splitlines()
+        explanation = [u('Full diff:')]
+    except Exception:
+        # hack: PrettyPrinter.pformat() in python 2 fails when formatting items that can't be sorted(), ie, calling
+        # sorted() on a list would raise. See issue #718.
+        # As a workaround, the full diff is generated by using the repr() string of each item of each container.
+        left_formatting = sorted(repr(x) for x in left)
+        right_formatting = sorted(repr(x) for x in right)
+        explanation = [u('Full diff (fallback to calling repr on each item):')]
+    explanation.extend(line.strip() for line in difflib.ndiff(left_formatting, right_formatting))
+    return explanation
+
+
+def _compare_eq_sequence(left, right, verbose=False):
+    explanation = []
+    for i in range(min(len(left), len(right))):
+        if left[i] != right[i]:
+            explanation += [u('At index %s diff: %r != %r')
+                            % (i, left[i], right[i])]
+            break
+    if len(left) > len(right):
+        explanation += [u('Left contains more items, first extra item: %s')
+                        % py.io.saferepr(left[len(right)],)]
+    elif len(left) < len(right):
+        explanation += [
+            u('Right contains more items, first extra item: %s') %
+            py.io.saferepr(right[len(left)],)]
+    return explanation
+
+
+def _compare_eq_set(left, right, verbose=False):
+    explanation = []
+    diff_left = left - right
+    diff_right = right - left
+    if diff_left:
+        explanation.append(u('Extra items in the left set:'))
+        for item in diff_left:
+            explanation.append(py.io.saferepr(item))
+    if diff_right:
+        explanation.append(u('Extra items in the right set:'))
+        for item in diff_right:
+            explanation.append(py.io.saferepr(item))
+    return explanation
+
+
+def _compare_eq_dict(left, right, verbose=False):
+    explanation = []
+    common = set(left).intersection(set(right))
+    same = dict((k, left[k]) for k in common if left[k] == right[k])
+    if same and not verbose:
+        explanation += [u('Omitting %s identical items, use -v to show') %
+                        len(same)]
+    elif same:
+        explanation += [u('Common items:')]
+        explanation += pprint.pformat(same).splitlines()
+    diff = set(k for k in common if left[k] != right[k])
+    if diff:
+        explanation += [u('Differing items:')]
+        for k in diff:
+            explanation += [py.io.saferepr({k: left[k]}) + ' != ' +
+                            py.io.saferepr({k: right[k]})]
+    extra_left = set(left) - set(right)
+    if extra_left:
+        explanation.append(u('Left contains more items:'))
+        explanation.extend(pprint.pformat(
+            dict((k, left[k]) for k in extra_left)).splitlines())
+    extra_right = set(right) - set(left)
+    if extra_right:
+        explanation.append(u('Right contains more items:'))
+        explanation.extend(pprint.pformat(
+            dict((k, right[k]) for k in extra_right)).splitlines())
+    return explanation
+
+
+def _notin_text(term, text, verbose=False):
+    index = text.find(term)
+    head = text[:index]
+    tail = text[index+len(term):]
+    correct_text = head + tail
+    diff = _diff_text(correct_text, text, verbose)
+    newdiff = [u('%s is contained here:') % py.io.saferepr(term, maxsize=42)]
+    for line in diff:
+        if line.startswith(u('Skipping')):
+            continue
+        if line.startswith(u('- ')):
+            continue
+        if line.startswith(u('+ ')):
+            newdiff.append(u('  ') + line[2:])
+        else:
+            newdiff.append(line)
+    return newdiff